a. Field of the Invention
Broadly speaking, this invention relates to radio communication. More particularly, in a preferred embodiment, this invention relates to a radio communication system wherein a fixed transmitter may selectively call any one of a plurality of portable receivers.
B. Discussion of the Prior Art
German Application No. 1,537,007 discloses a radio communications system comprising a plurality of portable receivers and at least one stationary transmitter wherein each receiver can be selectively called by means of binary encoded calling signals comprising pulses of equal length separated by intervals of differing lengths. In this known system, the receiver includes a first timing circuit having a first holding time for converting an incoming call signal into a binary signal, the length of said first holding time falling between the differing lengths of the pulse intervals; a memory for storing the binary signal characteristic of the particular receiver, each binary signal having a predetermined value; a counter for coordinating the received digits comprising the binary signal; a comparator circuit for comparing each digit of the received binary signal with the corresponding digit of the stored binary signal; and means for triggering a call indication when the received binary signal corresponds with the stored binary signal.
In the above described system, the particular suitability of the codes employed for transmitting calling signals over long, disturbed paths, on the one hand, and extremely short transmission paths, on the other hand, was not recognized.
The instant invention is based upon the discovery that it is possible to select the lengths of the encoded intervals as compared with the lengths of the transmitted pulses in such a way that even in situations where the pulses arrive lengthened or shortened, the two intervals can nevertheless be clearly distinguished. Only the complete loss of a pulse impairs the evaluation of a calling signal because in that event the call simply does not take place. It cannot, however, cause a false call because the requisite number of pulses to complete a call has not been reached. It is to be noted that non-completed calls cause much less subscriber inconvenience than false calls. In the prior art, however, false calls can be initiated by the chopping of individual pulses in the transmission path, thus causing multiple appearances of the same pulse at the receiver.